Overall, he added, the spice project has become a successful recipe for educating people, especially schoolkids, "that everything you do is connected to the watershed."

Illegal Drugs

The link from kitchen or bathroom to coast can also grease the path for some rather unsavory substances, such as illegal drugs, experts have discovered.

After a person has taken drugs such as cocaine, heroin, marijuana, and ecstasy, active byproducts of these substances are released into the sewage stream through that person's urine and feces.

These byproducts, or metabolites, are often not completely removed during the sewage-treatment process, at least in Europe, said Sara Castiglioni of the Mario Negri Institute for Pharmacological Research in Milan, Italy.

That means the drug-tainted wastewater can enter groundwater and surface water, which are collectively the major sources of drinking water for most people.

In a new review study, Castiglioni and colleague Ettore Zuccato found that illegal drugs have become "widespread" in surface water in some of Europe's populated areas.

For instance, in a 2008 study scientists discovered a byproduct of cocaine in 22 of 24 samples of drinking water at a Spanish water-treatment plant—despite a rigorous filtering and treatment process.

Likewise, in 2005, Zuccato found that a daily influx of cocaine travels down the Po River, Italy's longest river.

Though these drug traces are still tiny, it's possible that the potent residues could be toxic to freshwater animals, according to the study, which will be published in an upcoming issue of the journal Philosophical Transactions of the Royal Society A.

For this reason, the "risks for human health and the environment cannot be excluded," the study warns.

Pharmaceuticals

Scientists are also developing a clearer picture of how legal pharmaceuticals and personal-care products—from antibiotics and morphine to fragrances and sunscreen—are flooding our waterways.

For example, previous research had revealed that up to 44.1 pounds (20 kilograms) of pharmaceuticals flow down Italy's Po River each day.

Much like illegal drugs, traces of pharmaceuticals often filter through traditional sewage-treatment processes.

These products are also found in many U.S. waterways, and studies have shown that certain drugs may cause harm to the environment—though no evidence to date has shown effects in people, according to the U.S. Environmental Protection Agency.

Some of the drugs that mimic hormones, such as birth control, may also throw off an animal's endocrine, or hormone-regulating, system. Some male fish in the U.S., for example, have been growing female parts due to exposure to estrogen in the water.

Researching these substances is important, Castiglioni said, "because [these] are quite unknown contaminants, and they are present in the environment in huge amounts, especially for pharmaceuticals."

To control the flow of these substances, some experts have suggested creating "green pharmacies," which would allow a consumer to send back their drugs to a pharmacist or manufacturer instead of flushing them down the toilet and into the wild.

Contaminants

Current EPA regulations say that more than 90 contaminants must be filtered out of drinking-water systems, said Cynthia Dougherty, director of EPA's Office of Groundwater and Drinking Water.

Viruses and other microorganisms are kept at bay, as are inorganic substances such as lead, cyanide, copper, and mercury. Pollutants from fertilizer runoff, such as nitrate and nitrite, are also removed.

In addition, the agency regularly studies new chemicals that may need regulation. Of particular interest right now is perchlorate, a natural and human-made chemical used in fireworks and rocket fuel, Dougherty said.

At sufficiently high doses, the chemical—found in at least 4 percent of U.S drinking water—can reduce iodine uptake into a person's thyroid gland. If continued long-term, reduced iodine can lead to hypothyroidism, according to the agency, which is now seeking input on whether to regulate perchlorate.

Ultimately, "what you really want is to not ever have things you're concerned about in drinking water in the first place," Dougherty said.

That's why it's crucial for local communities to keep a close eye on what runs into their waterways, she said.

"If you have an understanding of what your source of drinking water is and what can happen to it," Dougherty said, "you can be a more educated citizen in engaging in those issues."